1. Field of the Invention
The present invention, generally, relates to switched-mode DC-DC power converters and, more particularly, to a new and improved circuit interconnected to substantially reduce the degrading effects of magnetization current on sensed load current.
To clarify what otherwise could be misleading nomenclature, the term "switched-mode" is used interchangeably with "switch-mode" within the technical field with which the invention relates, whereas in some patent areas, only the term "switch-mode" is found. In connection with the present description, the term "switched-mode" is used, but if a reader prefers, it may be read "switch-mode".
Similarly, a "DC-DC" converter is used as an intermediate element in many circuits that perform AC-AC, AC-DC, or DC-AC power conversion. Consequently, a circuit known as a "DC-DC converter" might be found in the art disguised in another form.
Also, it could be important to a reader of the present description to be aware of the distinction between a coupled inductor and a transformer, since an inductor is shown schematically the same as a transformer in electrical drawings and could be called, though erroneously, a transformer. A coupled inductor is used to store energy, while a transformer is not. The undesirable magnetization current of a transformer functions just like stored energy, but it would not be there if it could be eliminated, because it is deleterious to the control of DC-DC power converters.
A DC-DC converter is used customarily in conjunction with a control circuit to provide either a constant output voltage or one that is otherwise regulated from a voltage source that is subject to variations. A well known and useful technique used in the design of the control circuit requires an accurate sensing of the current in the inductor of the output LC filter, but this current usually is unreliable because of the degrading effect of transformer magnetization current.
A widely used technique to control DC-DC power converters requires a knowledge of its output current, and it is common practice to obtain it indirectly from the input current by sensing the current in the power transistors that switch the current in the primary of the power transformer. The current that is sensed in the power transistors is the sum of the magnetization current of the power transformer and the output current that is reflected to the primary.
Sensing this natural summation is useful for protecting the power switches from excessive currents, but in many practical instances, it is undesirable for control purposes due to the unstable effects on it by the transformer magnetization current.
The present trend toward miniaturization of electrical and electronic circuits compounds the problem further because miniaturization of power transformers often produces much more magnetization current than is desired. This increase in the magnetization current can compromise the design of the control circuit and lead to reduced performance.
Moreover, since the permeability of the core material of the power transformer is highly temperature-dependent, variations in ambient temperature can produce large changes in the magnetization current, making its control even more difficult. The effect of temperature on magnetization current is a complicated function, and this fact together with the usual wide range of manufacturing tolerance variations, a large range of input voltages applied and the customary variations in component values, serve to make any form of control loop of uncertain effectiveness. Normally, exceedingly complex electronic circuitry is required to achieve an acceptable performance level.
2. Description of the Prior Art
U.S. Pat. No. 4,162,524 to Jansson is entitled "DC-DC Converter" but describes only a technique for protecting the transistor switches of the converter that is responsive to the rate of change of current, not current magnitude. Therefore, the principle involved is at variance with the present invention.
U.S. Pat. No. 4,184,197 entitled "DC-TO-DC Switching Converter" to Cuk et al. describes a circuit for receiving only nonpulsating input current and does not use current sensing or control.
U.S. Pat. No. 4,680,687 to Stasch et al. is entitled "Switch-Mode Power Supply Having a Free-Running Forward Converter" and describes an arrangement to permit a switching transistor to be actuated only when the current is reduced to zero.